Multiproxy dendrochronological dating of coseismic land-level changes in the Puget lowlands

Abundant geologic evidence of past earthquakes in the Puget lowlands in Washington State raises substantial concerns about modern seismic hazards in the densely populated Seattle metropolitan and neighboring urban areas. Mapping and paleoseismic studies identify multiple faults across the Puget lowlands, including the Seattle fault zone, which ruptured and triggered catastrophic secondary effects such as landslides and tsunamis multiple times over the past millennia. There remain, however, substantial uncertainties on the recurrence of these hazards, their potential effects on coastal ecosystems, and their relationship to the nearby Cascadia subduction zone's plate interface. Large earthquakes can cause substantial land-level changes, including coastal subsidence and vertical offsets that dramatically change the local hydrology. These effects kill and preserve ancient forests through drowning and rapid burial, producing "ghost forests" and buried soils. Here, we use a multiproxy approach, including novel high-resolution ¹⁴C records from tree rings, traditional dendrochronology, and stratigraphic relationships to generate precise geochronology of coseismic forest deaths at multiple sites in the Puget lowlands. These data allow us to better understand the interplay of crustal faults with megathrust earthquakes and improve our understanding of earthquake recurrence beyond the limited span of instrumental records. Additionally, we generate multi-centennial, tree-ring records west of the Cascade mountain range that allow us to assess the impacts of climate in conjunction with extreme events on ancient coastal landscapes. These records will be available for other disciplines to calibrate their geochronologic records.